Natural manganese dioxide or electrolytic manganese dioxide produced from manganese ore is used as a material for a positive electrode for primary battery such as manganese dry battery or alkali dry battery. Of such batteries, the primary battery using electrolytic manganese dioxide has high performances as compared with a battery using natural manganese dioxide, and therefore, its demand is increasing year by year.
The electrolytic manganese dioxide is generally produced by the following steps.
A raw material ore containing manganese is dried and ground. The resulting particles are dissolved in a sulfuric acid aqueous solution to extract manganese in the sulfuric acid aqueous solution. The manganese-containing sulfuric acid aqueous solution obtained by this extraction step contains solid materials comprising a silcate which is insoluble in sulfuric acid. Therefore, at this stage, a first filtration is conducted to separate the solution into solid materials and an extracting liquid. However, elements such as iron, alkali metals or the like are dissolved in the extracting liquid. Therefore, those elements are subjected to refining step by oxidizing those using an oxidizing agent and neutralizing the oxidized products with lime stone to form those in the form of hydroxides, or in the form of sulfates, thereby precipitating those as solid materials. The extracting liquid thus treated is similarly subjected to a filtration treatment to separate the solid materials from the liquid, thereby obtaining a refined liquid containing manganese sulfate. The refined liquid thus obtained is then supplied to an electrolytic cell equipped with a titanium anode and a graphite cathode. Electrolysis is perfomed by applying current to deposit manganese dioxide on an anode plate. Manganese dioxide thus obtained is separated from the anode plate to obtain a flaky manganese dioxide. This flaky manganese dioxide is then ground and washed with water to obtain electrolytic manganese dioxide.
Solid materials formed in the above manganese extraction step and refining step are called manganese slags, and those manganese slags are all discharged into out-of-system. Those manganese slags are mainly constituted of water, Fe.sub.2 O.sub.3, MnSO.sub.4, SiO.sub.2 and Al.sub.2 O.sub.3. Those manganese slags discharged have conventionally been mixed with slaked lime (calcium hydroxide) for insolubilization treatment. The insolubilized slags have been dealed with disposal by land-fills.
However, with increasing consciousness to environmental problems in recent years, it becomes difficult year by year to secure the disposal land, and disposal by land-fills requires much costs. Therefore, it has strongly been demanded to establish a treatment method of manganese slag, which can treat the slag at low cost without giving load to environment.